NOx-Catalyzed Gas-Phase Activation of Methane:the Formation of Hydrogen

ournal of Natural Gas Chenistry 12(2003)90-92CIENCE PRESSNOr-Catalyzed Gas-Phase Activation of Methane:the Formation of HydrogenChaoxian Xiao. Zhen Yan, Yuan KouMAnuscript received March 0: 3. 2D)(K; revised April 15, 2003Abstract: NO, alyzed oxidlation of methane without a solid catalyst was investigAted, and a hydrogerselectivity of 27% was obt ained with an overall methane conversion of 31% and a free O2 concentration of1.7%at7o0℃Key words: gas-phase oxidation, methane activation, Imct hine. nitrogen oxide, hydrogen1. IntroductionOverall Factory). The amounts of NO. NO2, CHaCO. CO2 HCHO. CH OH and C2 H, were determinedGas-phase uxidation of methane without the use by in situ or on-line FT-IR usiug a Bruker Vector 22of any solid catalyst opens up new possibilities for spectrometer with a 10 cm long gas cell. The con-the direct conversion of methane to valuable chemi- centrations were automatically determiued using thecals. although results are still limited(1, 2 concerning OPUS Quantitative Method withpropriate calibthe production of oxygenates using NOr as the cata- tion curves. CO and H2 selectivity were calculated aslyst 13-9). In this paper, we show that the formation followsof significant amounts of hydrogen has always beenC(CO)finalignored in recent reaction and mechanistic studiesS(CO)Ha)initial-C(CHa)final×100%2. ExperimentalHS(H2 )=2(C(CHA)inital -C(CHA)#ina )*%The experiments were carried out at atmospheric3. Results and discussiempty quartz tube reactor (inner diameter 5 mm)The reactor was kept in a tubular furnace (20 cmVarying the CHa/O2 ratios shows that lowerlength). The gases were controlled by mass flow CH4/O2 ratios, i.c. higher O? concentration, in thecontrollers and premixed before entering the reac- feed gas lead to higher yields of H2. Figure I showsor with an N2 balance. The temperatures indicated that low yields of hydrogen are obtained in an oxyin this paper were the temperatures in the reaction gen atMosphere. When the O2 concentration is 4%zone of the reactor as measured with a thermocouple. (n(CH)/n(O2)=5), 52% of the oxygen is convertedThe anounts of H2. O2, CH4 and CO in the prod- the free O2 concentration is calculated to be 1.8% anducts were analyzed by on-line gas chromatography the yield of hydrogen is only 3%h, at 700C. When the(GC122 and 102G. Shanghai Analytical Instrument O2 concentration reaches 10%e (n(CHA)/n(O2)=2Corresponding author. Tel: +86-10-62757792; Fax: +86-10-62751708; Frmail: yuankou pku.edu. cnThe project supported by the Ministry of Science and Technology, China(G1999022202)中国煤化工CNMHGJournal of Natural Gas Chemistry vol. 12 No. 2 2003the Hl2 yield is about 10% at 700-800.C In that case, is less than 10% at n(CHa)/nO)=5 but higher thanthe O2 conversion is 83% and the free Oz concentra30%atn(CH4)/n(O2)=2(lAtn(CH4)/n(O2)=1,tion is calculated to be 1.7%. As the CH,/ O2 ratio 985y of the oxygen in the feed gas is converted, butdecreases from 5 to 2, the selectivity to H2 goes down the H2 vield does not improve the selectivity to Hy isslightly from 31%to 27%. The increase in H2 yield is a reduced to 16% and the calculated free O2 concentraonseqnence of the improved conversion of CH; which tion remaining in the reaction system is about 0.3%.H, yieldFigure 1. Effect of Oa(a)concentration and (b) conversion on H2 yield and selectivityReartion conditions: feed gas CH4 20%, NO 0 (6i%; How rate 40 ml/ min; reaction temperature 700"CThis rneans that low concentrations of free oxyIt is ccrtain that NOr plays a key role in the cat-gen, for example 1%-2%. have little adverse effect alytic conversion of CH4. The Hz selectivity increaseson the formation of H2. It is particularly important from 2% to 13% with a simultaneous increasc of CH4to notice that when frec O2 concentration is 1.7conversion when the no concentration changes from1.9%, the H2 concentration(carefully checked against 0.02%to 0.16%c under higher flow rates of 200 ml/mina standard curve)is 3.9%-4.5%, giving an H2/O2 ra-(employed in order to obtain reliable accuracy in thetio of about. 2measurement of NO concentration). The selectivityStudies with fow rates varying between 20 and to CO gors up faster than that of H2(Figure 3(a)200 ml/inin(residence times of 3.7 s-0.4 s) show that It is likely that thermal decomposition of CHA aya consistent H2 selectivity of around 26% can be ob- occur when the CHa/NO ratio is greater than 500,tained at 700-800'C for residence times in the range giving a high H2/CO ratio of about 4(line I in Fig-of 0.6-3.7 s(see Figure 2)ure 3(b)). As the CH/NO ratio goes down, the par-tial oxidation of ethane bccomes dominant(line IIin Figure 3 (b)), but complete oxidation of methane750℃to CO2 as a competitive sidc-reaction simultaneouslyoccurs( line Ill in Figure 3(b)). Aromatics are also ob-served in the products(toluene, etc. 2). This meanstheion conditions, gas-phasecatalytic oxidation of ethane can, in principle, beused to produce a variety of chemicals. This is likelyto be most valuable in those reactions where the prey-euce of a solid catalyst presents separation problemat the end of the reaction. Furthermore, because a00.51.01.52025303.540product derived from the activation of methane is al-Residence time(s)ways believed to be commensurate with the surfaceFigure 2. Effect of residence time on H selectivitystructure of a catalyst, gas-phase catalytic oxidationat different reaction temperaturesof methane may provide an excellent reference pointReaction conditions: feed gas CH4 20%,O2 10%, No 0.06% for comparing what is truly occurring ou the surface中国煤化工CNMHGChaoxian Xiao et al./ Journal of Natural Gas Chemistry Vol. 12 No. 2 20(irCO selectivity-H, selectivityline lILine lll51g00020.040.060080.100.120.140.6U0.020.040.060.080.100.120.140.16NO conFigure 3. Effect of NO concentn(a)CH, conversion, CO and H2 selectivity,(b)H/co ratioReaction conditions: feed gAR CHA I4%: flow rate 300 ml/ min: reaction temperature 700CPrevious reports in the literature do not giveany rationalization for the forination of hydrogenFitting curvevin NOr-catalyzed gas-phase oxidation of methaneBased on further consideration of the kinetic modelproposed by Bromly [10. it can be seen that severalelementary reactions may be responsible for the pro-duction of H36.10HNO+H→H2+NOHONO+H→H2+NO2CH4+H→CH3+HFigure 4. Correlation between the area ofv(C=O)IRpeak of HCHO and observed H2 corCH3NO2+H→CH2NO2+HReaction conditions: feed gas CH, 20%, O2 10%, NO 0.06%6After carefully checking the H2 concentration, theReferencesreaction rosults obtained at. different temperaturesand Aow rates indicate that, as shown in Figure 4, theincrease in H2 concentration parallels the increasc in1 Yan Z, Xiao C x, Kou Y Catal Lett. 2003, 85(1/2)129the v(C-O)peak area in the IR spectrum of HCHO (2) Yan Z Xiao C X Kou Y Catal Lett, 2003. 85(3/4)when H2 concentration is lower than 0.5%. This slggests that reaction (4)is most likely to be the ele- 3 Tabata K, Takemoto Y, Suzuki E. Catal Rev, 2012.mentary step responsible for the formation of H2. Onthe other hand. low yields of HCHO are always corre- 4 Layng T E, Soukup R Ind Eng Chem, 1928, 20: 1052lated with higher H2 concentrations, suggesting that 5) Mcconkey B H, Wilkinson P R Ind Eng Che Processthe decomposition of HCHO is also a possible routeDes dcv,1967.6;436for H2 production(4,8)G'Tabata K, Teng Y H, Yamaguchi Y J Phys Chem A000,104:2648HCHO+NO2→CHO+HONO(6) [7 Han L, Tsubota S, Ilaruta M. Chen Lett, 1995, 931(8 Otsuka K, Takahashi R, Yamanaka I J Catal, 199/9CHO+H→CO+H2185:182In addition, the gas-phase oxidation of mcthane [9] Teng Y, YamaguchiTakemoto T. Phys ChemChem phys,2000.2:3429to different chemicals by catalysts other than NOx [10) Brotnly J H, Barnes F J, Muris S, You x, Haynes Bopens up an intriguing interest for further studyS Combust Sci Tech, 1996. 115: 259中国煤化工CNMHG